The Introit was commonly troped or "farced" (from the Latin farci, or "stuffed") in the Middle Ages, as were the Kyrie, Gloria, and dismissals for Mass and the Office. When sung by a competent choir the Introit can be the most haunting moment of the Roman Mass, when the noise of the outer world is switched off and a noise, hallowed for its reservation to higher ears, resonates through the temple. It presents and makes present again the mystery and action of Christ commemorated that day. The psalm choices for the Introit in the more ancient Masses for feasts testifies as to just how real the Church of Rome understood these mysteries to be ("A child is born unto us...." "I am risen and still with you...." "Behold, the ruler is come...."). The farcing of the Nativity Introit, itself a variation of the medieval farcing of the Paschal Resurrexi Introit, creates a dialogue between heaven and earth in understanding the feast. The image of the vigilant shepherds seeking Christ born in a cave anticipates Mary Magdalen seeking Christ risen from a cave in the morning.
Quem quaeretis in praesepe, pastores? Dicite!
Christum natum infantem pannis involutum secundum sermonem angelicum!
Adest hic parvulus cum Maria matre eius, de quo dudum vaticinando Isaias dixerat propheta:
'Ecce, virgo concipiet and pariet filium!'
Et nunc euntes dicite, quia natus est!
Troped chants survived for a while after Trent. Standardized book publishing probably contributed more to the decline of troping than legislation.
While certainly an innovation I think tropes represent a legitimate liturgical development, a spontaneity that preserves what preceded it while adding something substantial for both the sake of worship and instruction.
Troping likely will not return, but the liturgical instinct behind must!